It is well known in the art of mobile radiotelephones to employ a receiver for receiving a receive frequency signal while a transmitter simultaneously transmits a transmit frequency signal in the other direction, the transmit frequency being separated from the receive frequency by a constant offset known as the duplex spacing.
Although the duplex spacing is nominally a constant, it can be a different constant depending on the frequency band in which the mobile phone is operating. Complications can then arise in constructing mobile phones that operate in more than one frequency band. Various approaches have been proposed for handling these complications.
One approach is to employ two separate and independent receiver-transmitter chains for each band of interest. This has the advantage of inherent redundancy for enhanced operation reliability and power control. However, the separate receiver-transmitter chains require extra space, resulting in a more expensive and bulkier mobile phone product. As the market demand shifts toward smaller, less expensive phones with more embedded features, this approach becomes less suitable.
U.S. patent application Ser. No. 08/795,930 entitled "Transmit Signal Generation with the Aid of Receiver" (Dolman) describes the use of the second local oscillator of the receiver as a reference frequency against which a transmit frequency is controlled relative to a receive frequency to achieve either a first or a second duplex spacing. The Dolman application is hereby incorporated by reference.
It is also known in the prior art, when packaging two synthesizer PLL circuits into a common integrated circuit, to synchronize or otherwise relate the reference dividers of the two PLLs so that their phase comparators do not mutually interfere. The Philips UM1005 and 8026 dual synthesizer integrated circuits available on the open market use this technique. These circuits include the use of fractional-N dividers and programmable loop bandwidth, such as described in U.S. Pat. Nos. 5,095,288 and 5,180,993, which are hereby incorporated by reference. Novel ways to employ such synthesizers in dual mode satellite/cellular telephones in order to achieve different tuning step sizes in different frequency bands are described in U.S. Pat. Nos. 5,535,432 and 5,610,559 which are also hereby incorporated by reference.
Another approach is to employ an offset voltage controlled oscillator (VCO) to mix up or down the receiver local oscillator frequency to generate the transmitter frequency. This approach is disclosed, for example, in commonly assigned U.S. patent application Ser. Nos. 08/675,171 and 08/823,068.
U.S. patent application Ser. No. 08/675,171, entitled "Dual Band Transceiver", describes the use of common radio components for dual bands. Sharing a common receiver local oscillator synthesizer, transmitter offset oscillator loop, transmitter UHF VCO, IF filter, and receiver IF circuit greatly reduces the number of components for a dual band mobile phone. However, the transmitter offset frequency is fixed for both bands. Thus, one band has to jog the main channel stepper to put the transmitter on frequency, i.e., one band has to change frequency for transmission. The band that must do the hop cannot, therefore, be used in full duplex mode, which is required for CDMA and multi-rate TDMA mobile phones.
U.S. patent application Ser. No. 08/823,068, entitled "Dual Band Mobile Station", also describes the sharing of similar common radio components between bands but enables duplex operation in both bands. However, as with all offset schemes, generation of the transmitter carrier is inherently troublesome because of the spurious performance of the transmitter. This is because mixing the synthesized VHF signal up with the receiver local oscillator frequency to the desired transmit band spawns up-conversion products which have to be filtered. These products may add a filtering burden to meet the transmitter output spectral mask requirements.
Continuous advances in electronics allow for smaller mobile phones complying with a variety of national and international protocols. The international mobile phone standard known as GSM in Europe and as PCS 1900 in the USA operates with a transmit/receive duplex spacing of 45 MHz in the European 900 MHz band; 95 MHz in the European 1800 MHz band, and 80 MHz in the U.S. 1900 MHz PCS band. The channel spacing is 200 KHz (13 MHz/65) and the transmitted symbol rate is 13 MHz/48. All timing in this standard is related to a 13 MHz clock, as is well known. The U.S. IS 136 system known as DAMPS operates with a 45 MHz duplex spacing in the US 800 MHz cellular band, and with an 80.4 MHz duplex spacing in the U.S. 1900 MHz PCS band, with a tuning step size of 30 KHz and a transmitted symbol rate of 24.3 Kilosymbols/sec. In IS 136, as is well known, the tuning step sizes and symbol rates and internal timing are all derivable from a 19.44 MHz clock. Yet another U.S. standard known as IS95 uses Code Division Multiple Access at a transmitted chip rate of 1228.8 MHz, with a duplex spacing of 45 MHz combined with tuning steps of 30 KHz in the 800 MHz band, alternatively 50 KHz steps combined with 80 MHz duplex spacing in the 1900 MHz band. In IS95, the chip rate and frequency step sizes are not easily derivable from the same crystal oscillator.
U.S. Pat. No. 5,471,652 discloses an arrangement including a multiplier which permits the operating point of a VCO in a single band phone to be adjusted for various bands. This patent does not discuss application to a dual band phone. In addition, according to this patent, a synthesized VHF signal (80-400 MHz) must be up-converted to the transmitter channel for digital (non-FM) operation. In the FM mode, the UHF oscillator is directly modulated and bled through the mixer.
It may be easily understood that combining two or more of the various abovementioned protocols in the same handheld unit is hindered by the variety of tuning step sizes, duplex spacings and symbol rates that must be synthesized. Consequently, there exists a need for an improved radio architecture to facilitate such combination.